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On Mon, 2022-05-09 at 07:47 +0200, Tomasz Moń wrote:
> The problematic case (and the reason why full/low-speed bus is
> mentioned) is the LINKTYPE_USB_2_0_FULL_SPEED. It is the case when
> you capture at the connection between full speed hub and the host
> (and possibly full speed device connected to a full speed hub if
> there arelow speed devices connected to the full speed hub).
The low speed packets (after the PRE packet) intended for low-speed
device connected to full-speed hub downstream port, do appear on the
link between full-speed hub and full-speed device.
> If there is low speed device connected to downstream hub port, then
> when the host wants to send packets to the low speed device, these
> will be sent at low speed to the hub. However, there will be PRE
> packet (sent at full speed) before every low speed transaction.
>
> Hopefully I have recently found full-speed only hub (thanks to Linux
> kernel bug report [1]), so I have all the equipment necessary to
> check how the OpenVizsla deals with the problematic case.
>
> [1] https://bugzilla.kernel.org/show_bug.cgi?id=213839
OpenVizsla does not handle this case. While the PRE packet is correctly
captured, the low-speed data is just exported by gateware as invalid
packet. Similarly, the LambdaConcept USB2 sniffer does not handle this.
LCSniff displays the "ERR/PRE Packet" colored in red (with no
indication of the low-speed data).
When OpenVizsla is configured to capture low-speed connection with the
capture cable connected between host and full-speed hub, the data is
just garbled. The only way to capture the low-speed data (with
OpenVizsla configured to capture low-speed) is to capture it between
the full-speed hub downstream port and low-speed device.
I am not really surprised that both OpenVizsla and LambdaConcept USB2
sniffer fail at this problematic case. This is a corner case really,
and it is not that much relevant nowadays where majority of hubs are
high-speed capable. When the high-speed hub is connected to high-speed
host, there is no problem with full/low speed packets as each
downstream port receives data at target device speed.
I am not sure if the capture problem can be solved with current
OpenVizsla and LambdaConcept USB2 sniffer design. If possible, then the
necessary changes would require gateware modifications.
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